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It is demonstrated that a grain size dependence exists for the rotational permeability of a series of MnZn polycrystalline ferrites, analogous to that predicted by the Globus model for wall permeability. To account for this behavior, a model has been developed which considers crystalline ferrite grains with intrinsic complex permeability, μi, surrounded by thin, nonmagnetic grain boundaries. The effectively measured permeability of the polycrystal (μe) is related in the model to the intrinsic permeability, the grain size ( D), and the grain boundary thickness (δ) according to the equation μe=μiD/μiδ+D. The almost linear dependence of permeability with grain size for fine-grained polycrystals emerges if one considers the limit where D is so small that D≪μiδ, and consequently μe=D/δ (providing δ remains constant). For large grains, where D≫μiδ, it is found that the model predicts a constant rotational permeability equivalent to that in a single crystal of the same material. In the situation where the intrinsic permeability of the ferrite displays a relaxational behavior and follows the Snoeks relationship, it is found that both the low-frequency permeability and the resonance frequency of the polycrystal are modified, but in a manner whereby the Snoeks relationship remains valid